Mechanical properties of friction-stir-welded polyamide sheets
© M. Husain et al. 2015
Received: 26 May 2015
Accepted: 19 July 2015
Published: 1 August 2015
In this study, the effect of processing parameters on the mechanical properties of friction-stir-welded polyamide (nylon 66) sheets is investigated.
Commercial polyamide sheets with the dimension of 250 × 150 × 8 mm are used in experimentations. The processing parameters considered in the welding include: rotational speeds in five levels and traverse speeds in three levels. Tensile and impact tests are performed in order to evaluate mechanical behavior of welded sheets. Full-factorial design of experiments and also analysis of variance was performed in present study.
Results show that when rotational speed increases, weld strength first increases and then decreases. In other words, there is an optimum rotational speed in which the welded samples could obtain the highest tensile Strength. Regarding the traverse speed, weld strength decreases with increasing of the traverse speed.
Obtained results show a significant relationship between considered properties and processing parameters through an analysis of variance (ANOVA) study.
The real plastics revolution in automotive industry began in 1950 when thermoplastics made their debut, starting with acrylonitrile butadiene styrene (ABS) and going on to polyamide (PA), polyoxymethylene (POM), and polycarbonate (PC) together with introduction of alloys and blends of various polymers. The ongoing development of advanced and high-performance polymers has dramatically increased their usage. Originally, plastics were specified because they offered good mechanical properties combined with excellent appearance, including the possibility of self-coloring. The application of plastic components in the automotive industry has been increasing over the last decades. Nowadays, the plastics are used mainly to make cars more energy efficient by reducing weight, together with providing durability, corrosion resistance, toughness, design flexibility, resiliency, and high performance at low cost (Szeteiova 2011). Friction stir welding is a new solid-state joining technique, which was originally developed and successfully applied for aluminum alloys. However, recently, attempts have been made to adapt friction stir welding technology to the joining of thermoplastic materials. In this solid-state joining process which welds the materials, the characteristics and properties must remain unchanged as far as possible (Shaikh et al. 2014). Many researchers investigated and formulated friction stir welding process which has produced structural joints superior to conventional arc welds in aluminum, steel, nickel, copper, and titanium alloys. Research and development efforts over the last decade have resulted in improvements in friction stir welding and the spin-off of a series of related technologies.
Sorensen et al. (2001) at Brigham Young University (BYU) studied the mechanical properties of the thermoplastics with friction stir welding (FSW) method using a hot shoe. In this study, they used non-threaded tools first and found that pin without thread causes a lot of tension to the device and a reduction in preserved material in the weld area. They concluded that increasing rotational speed and decreasing travel speed have positive effects on the weld strength.
Strand et al. (2003) at BYU, in another study in 2003, investigated the influence of parameters on flexural strength, tensile strength, and microstructure of the weld. Hot shoe was also used in this study. Experimental results showed that lower speed of travel and higher shoe temperature will lead to strength and better microstructure.
Saeedy and Besharatigivi (2010) have studied the feasibility of friction stir welding on medium density polyethylene blanks. The optimum welding condition has been determined. They have demonstrated that rotation speed and tool tilt angle have key roles in the seam elongation and strength, respectively. By applying this method of welding on polyethylene blanks, about 70 % of the base material strength is achieved.
Also, in other research, Saeedy and Besharati Givi (2011) studied the effect of varying process parameters (rotational speed, welding speed, and attack angle) on the weld quality of polyethylene sheets. A strength value of 75 % of that of the base material was achieved in their experimentation.
Aydin (2014) investigated the weld ability of polyethylene via friction stir welding method. In this study, three different heating processes were used. Welding processes are performed at room temperature, welding preheated plate samples at 50 and 80 °C with metal molding. A tensile strength of 72 % was achieved in non-preheated welds whereas tensile strength of parent material was achieved approximately at an optimum value of 89 % by pre-heating at 50 °C.
Anna Squeo et al. (2009) performed friction stir welding of 3-mm-thick polyethylene sheets with a cylindrical steel pin having two different pin diameters. Authors has concluded that even if process optimization is required, the final performances of the joints are sufficient to assess that friction stir welding of polyethylene may be a valid alternative to conventional joining technologies.
Arici and Selale (2007) investigates the effect of the tool tilt angle on friction stir welding of polyethylene. They showed that in welded samples, tensile strength decreases with increasing tool tilt angle. They also reveal that when tool tilt angle increases, the thickness of the welding zone decreases which in turn affects the tensile strength.
Payganeh et al. (2011) used friction stir welding for butt joining of polypropylene composite plates having 30 wt% glass fibers. The results indicated that tool pin geometry had a significant influence on weld quality, and the effects of rotational speed and tilt angle on weld appearance and strength were more than that of traverse speed.
Mendes et al. (2014) in their research work studied the effect of axial force, rotational and traverse speeds, the tool temperature on the morphology, weld quality, tensile strength, and tensile strain of friction-stir-welded acrylonitrile butadiene styrene sheets using a robotic system and a stationary shoulder tool developed to this purpose. They found that high axial force promotes the squeeze of the molten polymeric material, preventing introduction of air into the weld and helps cooling of the weld, avoiding shrinkage, and voids formation. Also, they observed that high axial force improves tensile strength and strain of welds. In addition, it is reported that the rotational speed is primarily responsible for heat generation, promoting adequate plasticizing, and mixing the polymer, and high tool rotational speed improves tensile strength and strain of welds.
Kumar et al. (2012) in their experimental research work studied the welding force of friction stir welding of AA5083. Their results show that tool rotational speed, welding speed, and tool shoulder diameter are the most significant parameters affecting axial force. They also showed that longitudinal force is significantly affected by welding speed and probe diameter.
In present research work, friction-stir-welded butt joints of polyamide sheets are obtained using a cylindrical tool in various welding conditions. The aim of the study is to investigate the effect of friction stir welding parameters (rotational and traverse speeds) on the mechanical properties of polyamide sheets in order to obtain the optimum condition that leads to a weld with the highest quality.
Materials and tool design
Levels of selected parameters in present study
Rotational speed (RPM)
Traverse speed (mm/m2)
At the start, the FSW tool is plunged into the sheets until it touched the shoulder surface. After few seconds as the dwelling time of preheating and softening of the material, the tool starts to moves along the weld line. Since the tool is removed when it reached to the end of the line, a hole is created at the end of the joint interface (Panneerselvam and Lennin 2014).
Result and discussion
Mechanical properties of welded specimens
Rotational speed (rpm)
Traverse speed (mm/min)
Tensile strength (MPa)
Relative tensile strength (%)
Impact strength (kJ/m2)
Relative impact strength (%)
Effect of processing parameters on tensile strength
Another important result that could be obtained from Table 2 is the fact that the maximum relative tensile strength of the welded samples is about 54.66 % of the base material that occurred in a specimen welded with rotational speed of 1570 rpm and traverse speed of 42 mm/min. Also, the minimum strength (about 6.94 % of the base material) occurred in a sample welded with rotational speed of 780 rpm and traverse speed of 62 mm/min.
Also, as Fig. 5 depicts, welding with higher traverse speed reduces the tensile strength. This could be because of the fact that in higher traverse speed, faster movement of tool leads to poor mixing of the material. In other words, welding with smaller traverse speed allows the two sheets to get enough time for mixing and homogenization. Therefore, weld line deflection and deformation in samples with higher traverse speed lead to poor material mixing and consequently weak tensile strength.
ANOVA table of tensile strength
Effect of processing parameters on impact strength
One interesting result that is obtained from Fig. 6 is the fact that as the traverse speed increases, the optimum rotational speed is almost constant. In other words, regardless of traverse speed, an optimum rotational speed (i.e., 1570 rpm) can be used for welding polyamide sheets. Nevertheless, if the traverse speed increases, the impact strength of welded samples decreases. This could be because with high traverse speed, the melted material will pour out of the welding area. Additionally, in a high travel speed, high value of temperature could not be created. Therefore, poor mixing of the melted material will happen, and consequently a poor quality weld is obtained.
Similar to the results of tensile test, another important result that could be obtained from Table 2 is the condition in which the relative impact strength could be maximized. The maximum relative impact strength of the welded samples is about 29.48 % of the impact strength of base material which is obtained in a specimen welded at 1570 rpm rotational speed and 42 mm/min traverse speed. Moreover, the minimum impact strength is about 5.76 % of the base material, which occurred in a sample welded at 780 rpm rotational speed and 62 mm/min traverse speed.
Another interesting result that could be concluded from Fig. 6 is that in low rotational speed (lower than the optimum rotational speed or 1570 rpm), impact strength (and also tensile strength in Fig. 4) will decrease with increasing traverse speed. But at high rotational speed (higher than 1570 rpm), this behavior could not be seen. In other words, at higher rotational speeds, traverse speed of 42 mm/min could be chosen as optimum traverse speed in which the mechanical properties of welded sheets could be maximized.
ANOVA table of impact strength
In this paper, friction stir welding process is used to weld polyamide (nylon 66) sheets. As mechanical properties, tensile and impact strength of friction-stir-welded polyamide sheets are studied under varying processing parameters. Results show that welding strength increases when rotational speed increases. However, further increases in rotational speed (higher than 1570 rpm) will decrease the welding strength. Regarding the traverse speed, it is concluded that welding strength decreases when the traverse speed increases. The maximum tensile strength of the welded samples was obtained about 8.51 MPa. Since the tensile strength of the base material is 15.57 MPa, the maximum relative strength of the joint was concluded approximately 55 % in comparison with the base material. Impact strength of the base material was also obtained 36.6 kJ/m2, while at the optimum conditions, the maximum amount of impact strength was achieved about 10.8 kJ/m2. It shows that relative impact strength of the best weld was approximately 30 % compared to base material.
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